FIG. 1 is a diagram illustrating a problem that may occur when handover is performed in a mobile communication system according to the related art.
In a mobile communication system, handover means a procedure that a user equipment (UE) which is connected with a source base station to perform communication is disconnected from the source base station as the user equipment moves and establishes a new connection with a target base station to which the user equipment moves. The user equipment can receive seamless services from a network during movement through the handover procedure.
If the handover procedure is initiated, the source base station needs to transfer some data blocks stored therein to the target base station, wherein the data blocks should be transmitted to the user equipment which performs handover. The target base station transmits the data blocks transferred from the source base station to the user equipment after completing the handover.
In FIG. 1, a data block ‘S’ means a service data unit (SDU), and a data block ‘P’ means a protocol data unit (PDU). Also, ‘60’ in ‘S:60’ or ‘P:60’ means a sequence number (SN) of the SDU or the PDU. A specific protocol layer of the source base station, for example, a radio link control (RLC) layer receives service data units (SDUs) delivered from a higher network node or a higher layer to generate protocol data units (PDUs), and transmits the generated PDUs to the user equipment.
If handover is performed from the source base station to the target base station due to movement of the user equipment, the source base station should transfer, to the target base station, data blocks which are not sure that the user equipment has successfully received them and all data bocks of the source base station thereafter. In FIG. 1, the source base station transmits the data blocks up to ‘S:65’ or ‘P:1006’ to the user equipment. At this time, with respect to PDU of ‘P:1002,’ it is supposed that ACK has not been received from the user equipment or NACK has been received from the user equipment. In this case, if handover is initiated, for example, if the source base station transmits a handover command (HO command) message to the user equipment, the PDUs from ‘P:1007’ are not transmitted to the user equipment.
Since the source base station has failed to receive ACK from the user equipment or has received NACK from the user equipment with respect to PDU of ‘P:1002,’ the source base station or the target base station cannot identify whether the user equipment has successfully received ‘S:61’ and ‘S:62’ SDUs included in the PDU of ‘P:1002.’ Since the source base station cannot identify information of the data blocks which the user has successfully received, the source base station transmits, to the target base station, the data blocks which are determined that the user equipment has failed to successfully receive them, i.e., all data blocks from ‘S:61.’ The target base station transmits, to the user equipment, the data blocks from ‘S:61,’ which are received from the source base station, after handover ends.
In this case, for example, if the source base station has failed to receive ACK or has received it as NACK even though the user equipment has transmitted ACK to the source base station after successfully receiving PDU of ‘P:1002,’ transfer of the data blocks from the source base station to the target base station and transmission of the data blocks from the target base station to the user equipment after end of handover may correspond to unnecessary transmission. For this reason, problems occur in that such unnecessary transmission of the data blocks increases network congestion and deteriorates efficiency of radio resources use.